Improved ForceAtlas2

Hi there. This new version should still be tested, but it is far more fluid and efficient in every version I have tested. The improvement is very visible.

It might be branded "ForceAtlas 2.1" or some different name, though. Only the adaptive speed has been changed, but some results might be different due to convergence issues. It worked very well on all the various graphs I use for tests, but some specific graphs might not converge contrary to ForceAtlas2. I did not deal with this since I was not very efficient at understanding how the plugins are integrated. I just modified the algorithm itself.

Also, because more and more computers deal well with multiple threads, I updated the defaut setting from 2 threads to 4 threads.